Liquid crystals are used primarily as dielectrics in indicating devices, since the optical Properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the Person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells ("super-twisted nematic"), SBE cells ("super-birefringence effect") and OMI cells ("optical mode interference"). The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical and thermal stability and a good stability towards electric fields and electromagnetic radiation. Further, at the usual operating temperatures they should have a suitable mesophase in as broad a range as possible (for example a nematic or a cholesteric phase for the cells referred to above), but nevertheless should have a sufficiently low viscosity and in the cells should give rise to short response times, low threshold potentials and a high contrast. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfill different requirements depending on the field of application and type of cell. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy and an electrical conductivity which is as low as possible.
In order to fulfill these to some extent contradictory requirements, it is generally necessary to produce mixtures having up to about 15 components. It is therefore important that the components have a good miscibility with one another and a sufficient solubility.
In order to achieve sufficiently broad mesophase ranges, it is usually necessary to add clearing point-increasing components to the mixtures, but in this case the viscosity and the electro-optical properties can be influenced disadvantageously. Further, materials having a low optical anisotropy, which are of interest e.g. for actively-addressed liquid crystal indicators, frequently produce smectic tendencies and usually also lead to an increase in the threshold potential, in the viscosity and/or in the response times. Furthermore, non-polar materials having a high optical anisotropy often have only smectic mesophases or even no liquid crystalline properties.